The present invention relates to fiber optical devices and lasers, and more specifically, to mode-locked fiber lasers.
Ultra short optical pulses can be used in a number of applications including optical information processing and data communication, optical probing with high temporal resolution, laser surgery, and material processing. In particular, recent advances in optical data communication with data rates up to 2.5 Gbits/s or higher demand compact ultra fast light sources with low maintenance, high reliability, and low cost.
Fiber lasers have been developed as a new generation of compact, inexpensive and robust light sources. In essence, a fiber laser is an optically-pumped resonator with a doped-fiber as the gain medium. As the gain exceeds the total optical loss in the resonator, a laser oscillation can be generated. Many different dopants can be used to achieve laser oscillations at different wavelengths. Atomic transitions in rare-earth ions can be used to produce lasers from visible wavelengths to far infrared wavelengths (e.g., 0.45 xcexcmxcx9c3.5 xcexcm). Er-doped fiber lasers for producing optical pulses at 1.55 xcexcm are particularly useful for optical fiber communication since the optical loss in the commonly used silica fibers is minimum at about 1.55 xcexcm.
Mode-locked fiber lasers can use various cavity configurations such as linear, ring, and figure-eight geometries. See, for example, U.S. Pat. No. 5,008,887 to Kafka et al., U.S. Pat. No. 5,513,194 to Tamura et al. However constructed, a mode-locked fiber laser is configured to have multiple longitudinal modes that oscillate simultaneously. A mode-locking mechanism is implemented in the resonator to synchronize the phases of different modes in such a way that the phase difference between any two adjacent modes is a constant. These phase-locked modes constructively add to one another to produce a short pulse.
Two common mode-locking schemes are active mode locking and passive mode locking. Active mode locking modulates either the amplitude or the phase of the intracavity optical field at a frequency equal to one or a multiplicity of the mode spacing. Active mode locking can be implemented by using intracavity electrooptic and acoustooptic modulators.
Alternatively, passive mode locking uses at least one nonlinear optical element inside the resonator to produce an intensity-dependent response to an optical pulse so that the pulse width of the optical pulse exiting the nonlinear element is reduced. Compared to active mode locking, passive mode locking can be used to achieve shorter pulses and therefore can be used advantageously to produce ultra short light sources. Commonly used passive mode locking techniques include saturable absorbers, nonlinear fiber-loop mirrors (e.g., figure eight fiber lasers), and intensity-dependent nonlinear polarization rotation. See, Richardson et al., Electronic Letters, Vol. 1, pp. 542, 1991 and Tamura et al., Electronic Letters, Vol. 28, 2226, 1992.
Mode-locked fiber lasers are much more compact and reliable than solid-state mode-locked lasers such as color-center lasers and Ti-Sapphire lasers. Compared to mode-locked semiconductor lasers with typical pulse widths of 10-20 ps and peak power of milliwatts, mode-locked fiber lasers can generate shorter pulses with higher output peak power.
The present disclosure describes a passive mode-locked fiber laser with a simple linear cavity and a saturable absorber to generate femtosecond pulses with a peak power up to and greater than tens of watts.
A mode-locked fiber laser of the invention generally includes an optical resonator defined by first and second optical reflective elements, a pump light source which provides a pump beam at a selected pump wavelength or within a specified pump spectral range, a doped fiber gain medium disposed in the resonator responsive to the pump beam to produce an optical gain at a laser wavelength within a laser gain spectral range, a pump optical coupler disposed to couple the pump beam into the doped fiber, and a saturable absorber disposed relative to the second reflective element that produces an intensity-dependent absorption at the laser wavelength.
The doped fiber gain medium and other fiber links within the resonator are preferably made of polarization-maintaining or polarizing fibers that are aligned with one another along a polarization axis. This keeps the laser polarization parallel to a principal axis of the fibers without using additional polarization controlling devices. This polarization-maintaining configuration simplifies the construction of the resonator and allows for a reliable long-term laser operation without need for polarization maintenance.
The saturable absorber preferably exhibits a slow saturation process and a fast saturation process. The slow saturation process has a low saturation intensity and can be used to initiate mode locking when the intracavity intensity fluctuates at a low power level. As the pulse intensity builds up, the pulse width can be further reduced by the fast saturation process. The slow saturation process allows use of long-lasting low-power semiconductor light-emitting devices such as LEDs and laser diodes as the pump light source to achieve a reliable operation up to the life time of these light sources. Many semiconductor compounds have such slow and fast saturation processes that are originated from inter-band and intra-band transitions and can be used to implement the invention.
The pump light source may include a light-emitting element such as a LED and a laser diode to produce pump light at one or more pump wavelength in resonance with at least one optical transition in the doped fiber gain medium for producing photons at the laser wavelength. The light-emitting element can be electrically controlled to produce an adjustable output power.
The pump optical coupler may include a wavelength-division multiplexer that couples the pump light into the doped fiber gain medium. The pump light is preferably coupled into the resonator to propagate in a direction away from the saturable absorber to reduce the amount of the pump light into the saturable absorber, thus reducing any optical damage to the absorber by the pump light.
Wavelength-selective optical elements such as gratings and bandpass filters can be incorporated into the resonator to effect a frequency tuning mechanism. This produces tunable ultra short optical pulses within the gain spectral profile of the doped fiber medium.
A mechanism for tuning the pulse repetition rate can be further included by changing the optical length of the resonator in a controllable manner. A fiber stretcher or a positioner may be used for this purpose. In addition, a feedback loop may be implemented to lock the pulse repetition rate to an external clock. A portion of the output pulses is detected by a photodetector. An error signal can be electronically generated to indicate the relative delay of the pulse rate and the external clock rate. A control circuit sends a control signal to adjust the length of the resonator by, for example, changing the position of the first reflective element, to reduce this error signal.
A fiber laser in accordance with the invention may be configured to produce either transform-limited soliton pulses or non-soliton pulses. The soliton operation can be achieved by adjusting the cavity parameters so that the group-velocity dispersion and the nonlinear self-phase modulation balance each other. Conversely, the laser may be adjusted to produce non-soliton pulses as desired.
In addition, a fiber laser in according to the invention can be configured to significantly reduce noise and timing jitter in the output pulses. Implementation of the polarization-maintaining configuration can reduce or minimize the noise and jitter caused by variations in the light polarization. The reflection at the pump wavelength within the resonator can be reduced or minimized by using anti-reflection coating at the pump wavelength on any optical surface, using angle-polished fiber facets, or using bandpass filtering elements that block light at the pump wavelength.
One advantage of the invention is the simplicity of the linear resonator in a polarization maintaining configuration. Another advantage is the capability of tuning the pulse wavelength. Yet another advantage is the capability of tuning the pulse repetition rate.
These and other embodiments, aspects and advantages of the invention will become more apparent in light of the following detailed description, including the accompanying drawings and appended claims.